Electro-optical system



Dec. 1, -1942,. F GRAY ELEcTRo-oPT1cAL-sYsTEM Filed March 4, 1936 l'3 Shee's--Sheetl 1 /N VEA/TOR By F. GRAY @6% A7' 'ORNEV Dec. 1, 1942.

Filed Maroh-4, 1936-' F. GRAY ELECTRO-OPTICAL SYSTEM 3 Sheets-Sheet i?.L

@y i 7 I.

rcw/VEV Dec. 1, 1942. F, GRAY ELECTRO-OPTICAL SYSTEM Filed March 4, 1936 5 Sheets-Sheet 3 INVENTOR By ww m. *0,30% uzhl .Lik

\ A TORNEV,

Patented Dec. l, 1942 Y ELECTRO-OPTICAL SYSTEM Frank Gray, New York, N. Y., aslignor to Bell Telephone Laboratorien Incorporated, New

York, N. Y., a corporation of New York Application Maron 4, 1ers, serial No. 61,051-

iz claims. (ci. 11s-1.a)

This invention relates to electro-optical systems and more particularly to means for setting up current variations representative of the various light-tone values of an object, as in television scanning.l

It has been proposed heretofore, in transmi ting an object to a distant station, to scan the object itself with a moving beam of light. Each element of the object, therefore, receives light from the scanning beam for only a very short period of time so that the photoelectric current produced by the light reflected from the object is relatively small. To remedy this situation, .it

- has also been proposed to continuously illuminate the entire image field and reflect the light variations onto Vone surface of a lightsensitive target or screen located in a specially con-v structed cathode ray tube, the cathode ray of the tube being adapted to scan the surface of the screen with a pencil of electrons. Due to the fact that the light sensitive screen must be placed within the cathode ray tube in order'to utilize for scanningpurposes the moving pencil of electrons constituting the cathode raybeam,

the special cathode ray tube is very expensive and as these tubes must be, replaced frequently,

this expense is no small item.

It is an important object of this invention to A provide a novel, compact and relatively `inexpensive light sensitive. electric unit toward which an image of the entire object is directed and which is scanned by a moving beam of radiant energy from a source without thecontainer enclosing the unit.

A further object is to provide a novel means I2 for translating light variations into electric current variations, as for example in the production of image currents in television systems, making use of a relatively thin plate-like element of photo-emlssive material for generating a moving beam of electrons for scanning another photoemissive element upon which the light variations are impressed.

A further object of this invention is to provide a novel supporting member for a light sensitive electric element which permits aow of current therethrough with negligible lateral spreading.

These objects are accomplished in the preferred embodiment of the invention; which is herein@- t after described in detail, by providing a scanning device comprising a gas-tight container 'enclosing a mesh electrode.v a screen or target comprising a thin," high resistance supporting plate having its face adjacent the grid coated scanning control member comprising a transparent plate closely adjacent the supporting plate and carrying on its face adjacent this plate a transparent metallic layer or iilm carrying a layer of photo-emissive material. Associated V with this device and in front thereof is an'optical system for` directing-radiations from a complete field of view through the interstices of the mesh electrode onto the photo-emissive coating of the high resistance supporting plate, whereby the elemental areas of this coating are activated in accordance with the light-tone values of correspending elemental areas of the'iield of view. On the opposite side of the device, that is, be-

yond` the rear face of the transparent plate and v outside the container, there is provided means for producing a moving beam ofradiations which beam traverses in succession elemental areas of the photo-emissive layer carried by the transparent plate, whereby this layer is activated to cause the production of an intense moving beam of electrons'which scans in succession the corresponding elemental areas of the photo-emissive coating carried bythe high resistance supporting plate. The transparent metallic4 conducting iilm is connected to the fine mesh electrode by an external circuit including in series a direct current source and a resistance. The electronic scanning beam operates to cause small areas of the photo-emissive coating to emit, in

succession, electrons corresponding to the inten-V sity of the radiations received from the respective elemental areas of the object or field of view, whereby there is caused to flow, through the ex- 'ternal'circuit and the resistance included therein,

an image current which is representative of the light-tone values of the various elemental areas of the neld of view. The resistance may be connected to the input circuit of an amplifier having its output circuit connected to a transmission circuit or radio system. The moving beam of radiations for controlling the production 'of the electron scanning beam may be produced by an optical means including an apertured scanningdisc associated with a suitable light source. A

cathode ray tube may be used for this purpose,-

in which case such tube may be, and is prefer-1 ably, a separate entity `from the apparahxs in:

ciuding the target or screen, its associated electrode and the photo-emissive scanning member.-

The invention will be more readily understood by referring to the following description taken in connectionwith the accompanying with a layer of photoemissive material. and a 55 forming a part thereof. in which:

Fig. 1 is a diagrammatic representation oi a television system involving this' invention;

Fig. 2 is a detail view of the assembly within the scanning tube:

Pig. 3 is a side view of the assembly of Fig. 2;

Fig. 4 diagrannatically illustrates a second television system involving the invention;

Fig. 6 diagrammatically illustrates the lateral spreading oi current in the case where the current admitted to a thin nlm is free to emerge over the entire opposite sideot the nlm:

-Fig. 6 shows the relation Abetween currentk ldensity and distance from the edge of the scan- -T,` suitable connecting means L, and a receiving station R.

The transmitting station T includes a scanning device S, an optical system for forming an image oi an object or neld of view on a coating ofphoto-emissive material forming an element oi' a target or screen C in the scanning device S, and optical means for producing a moving beam oi radiations for scanning a photo-emissivelayer 'l oi' a composite plate or scanning control member designated as M, the scanning control member M being also located in the scanning device S.

The 'scanning tube S comprises a gas-tight container I enclosing, as shownin greater detailin Figs. 2 and 3, an assembly including a nne mesh or other light transmitting anode 2 oi nickel gauze or other suitable material, a composite screen or target C, and a scanning control member M. The electrode 2 and members C and M` are of substantially -the same size'. and are mounted parallel to each other with a very small space between the members C and M. -The However, ii.' this plate is suniciently thin, no special precautions need be taken to thus decrease its lateral conductivity.

In order 4to fully understand how lateral conductivity may be made negligible byxnaking the nlm l very thin, two cases oi current now through a thin nlm will be considered.

'I'he nrst case or example illustrates the situation where current is admitted over only an ele-l mental area on one side of the nlm and is free to emerge over the entire opposite side of the distance between these members is preferably of the order of the dimensions of an elemental area denned by the size o! the scanning spot. 'I'he composite target or screen C, yupon which an image oi' an object or neld of view O is formed,

4 preferably comprises a high resistance insulating plate and supporting member i having a coating 4 of photo-emissive material, which coating is adjacent the mesh electrode l. The supporting plate 3 is quite thin and comprises a material-of very lowA conductivity,- practically an insulator, so that nlm, as shown in Fig. 5. 'I'he lateral-spreading oi? current in the nlm thus causes an enective dinusion of the elemental area. It the nlm is thin compared to the diameter of the elemental area, the spreading is approximately the same as if the perimeter were a straight line. Let it be assumed that s is the thickness of the nlm and, that :c and y are coordinates measured from the edge -oi the elemental area along the nlm and transverse to the nlm, respectively, as shown in Fig. 5. There are two coniugate potentials and gb Awhich denne the now of current in the nlm and which obey the equations e cos ay=sin be cosh w where D is the current density and k is the conductivity of the material.

In Fig. 6, this current density is plotted for.

various distances from the edge of theelemental area, the distance being given in terms of the nlm thickness s. It is seen by examining the curve shown in Fig. 6 that the now of currentis substantialLv connned within a region extending about twice the nlm thickness beyond the edge oi' the elemental area. The diffusion of a spot due to lateral conductivity'will, in the case under the lateral conductivity (along the' plate fromv elemental area to elemental area) is low compared to the conductivity through the plate over an elemental area. This property will be discussed more fully below. 'I'his plate 3 may consist of conducting glass, that is, glass which is slightly conducting. or ordinary glass impregnated with metallic particles, or other suitable high resistance material such as an alloy or mica and copper. The photo-emissive coating l may consist or a thin layer or nlm o! atomic thickness consisting of any suitable photo-emisslve material, such, for example. as is employed in. commercial caesium, potassium, or photoelectric cells. The plate 8, which is opaque, may be cross-hatched with closely spaced lines or otherwise treated to decrease its lateral conconsideration, be about equal to twice the'nlm thickness. For a nlm whose thickness is, for example, one-tenth the diameter of an elemental Enea, this will not be a serious amount oi spread- For the case under consideration, the shape of the lines ot current now is independent of the conductivity oi the material. ior the Equations l do not contain the conductivity k of the material. -'111e spreading is thusindependent of the material and depends only on the thickness oi' the nlm.

'I'he second and more critical case arises v. ier. current is admitted over only an elemental are on one side of the nlm and cannot emerge over the area directly opposite (as this area corresponds to a dark spot on the object), but can emerge over the entire remaining portion of the latter side of the nlm, as shown in Fig. 'l'. In this case, it is important that the current be small.

l for, under ideal conditions. there wauld'be no ductivity from elemental area to elemental area.

now of current.

An idea of the required constants of the nim may be obtained in the following manner. with refermcetol'ig.8,letnbetheradiuso`theele mmtal area over which currentis admitted and letnbetheradiusofaportionoftheiilminside ofthiselementalarea. Then,theresistancefrom the latter region to the region outside of the eiementalareaislesthanthe lateral resistance of the ring-like section of the iilm. The ring-like section will have alateral resistance equal to 21u-ra 2n wherepisthespeciiicresistanceoftheiilmand a is its thickness. When current can emerge directly opposite, the resistance through the lm is log Inasmuch as it is desirable to limit the lateral ilow'of current, the resistance R2 must be so large that the current iiow is limited because of the large potential drop across this resistance. If E is the series electromotive force, the maximum current that can flow from the region of,

radius n is i- (5) Suppose that it is demanded that i be one-tenth of the total normal current from an elemental area when is equal to 0.9. Then taking the normal current as 1 amperes which is a reasonable and practical .value for television currents, and taking E as 10 volts, lt is seen that Ra should be about 1012 ohms. This can be expressed as p g 1 l '2; 10g :6 10u For a film x10-s cm. thick, this requiresthat p=3.44x1011 ohms/cm.:i (7) specic resistance. In general, then, a material having a speciiic resistance of the order ofi()u ohms/cm.3 will be desirable for satisfactory operation for a. picture current of 101 amperes. If a current of 10-s amperes is obtained, the speciiic resistance of the material needonly be of the order of 10a ohms/cm?. It should be understood, however, that these iigures are only examplesv and are not limits.

` that the speciiic resistance of the material should preferably lie within the range from 101 to 10 times the reciprocal of the average image current. The iilm thickness should be preferably one-tenthor less o! the size of the scanning spot.

The scanning control member M has for its function the generation of a movingbeam or electrons which beam is used to scan the successive elemental areas of the ccmpite target or screen C. This moving beam of electrons is preferably generated by a moving beam of light produced by an apparatus outside the scanning device S. The scanning control member M preferably comprises a glass plate G on which is mounted a transparent metallic nlm 8 of silver or similar metal, on which is mounted a 'nonconducting, photo-emissive layer 'l of atomic thickness. By making the photo-emissive coatyings 'br layersvery` thin. there is emission only from the elemental area being scanned as the layer is thus made substantially non-conducting laterally thereof. Another way this can be done is to make the layer of a number of discrete globules.

The conducting transparent metallic layer I is connected to the electrode 2 by ari-external circuit, including a battery l in series with a resistance l. The latter is included in the input circuit of an ampliiier Il. The negative terminal of the battery is connected tothe transparent metallic layer C, and an intermediate point thereof is connected through an extremely high resistance Il to the opaque resistance plate I, whereby this plate is positively polarizedwith `respect to the transparent metal backing 0 of the photo-emissive layex` l, and somewhat negatively polarized with respect to the electrode 2. the positive terminal of battery Ii being connected through the resistance l to the anode 2..

The arrangement is operative, however, if the connection from the intermediate point of the battery l is omitted, the plate I then tending to assume a potential intermediate that of the transparent metallic layer i and the mesh electrode 2.

The object or iield of view O la4 illuminated by a source of radiations (not shown) and an optical system represented generally by the lens I2 gathlers radiations reflected by the object or neld oi' view O and directs themthrough the interstices 'of the electrode 2 upon the .photo-emissive coaty in accordance with the tone values of corresponding lof-"the supporting plate 3. The elemental areas of the coating 4 are thereby illuminated ing elemental areas or the field of view O. There will be some electronic emission to the mesh elecsumming up, it 1s clear that the mm s should ance should not be too high. however, as the operation of the device depends upon the conduction of electrons transversely therethrough. It will be appreciated that with so many varitrede 2, but because of the low capacity between the coating l and the mesh electrode 2, a static lcondition is quickly reached. Due to the `hi8h` lateral resistance of the plate I, no more electrons will ilcw tothe mesh electrode 2 except those later released under the influence of the scanning control member M.

Associated with the device S is a second optical means adapted to control the operation of the photo-emissive scanning control member M which optical means may comprise a source of radiations IS, a lens system, represented generally by a lens Il, for gathering radiations from the source Il and applying them in the form of a beam I5 toward a rotating disc I B provided with a series of apertures a arranged near its periphi apertures a pass through once per revolution and in a time interval within f the period of persistence of vision.

The moving beam 'of radiations B passes through the transparent plate G and the transparent metallic layer l to successively scan elemental areas of the photo-emissive layer l. which stage ampliiier. over the line L receiver R. For sion,theamplified imagecurrentwouldbeused to modulate acarrier output of which includes a glow cooperates with a rotatingdisc-II, of apertures arranged in a spiral and be driven in synchronismand in phase lam ha directly or after amplification if found necesareas are thereby activated to emit electrons and. since the insulating plate l has a positive polarity with respect to the layer "l,l there results a moving beam of electrons which scans the sary. Any suitable receiver may be used. l'br example, a satisfactorylreceiver is disclosed in U. B. Patent 1.728.122, september 10, 1929, to Horton, which may include a glow lamp of the type disclosed in U. s. Patent 1,918,309, July i8, 1933. to Weinhart, provided with a gas charge of the character disclosed in U. E. Patent 1,871,266,

August 9, 1932, to Gray.

For maintaining the discs Il and t! in synchronism and in phase, any suitable system may opaque high resistance plate i. Because of the shortdistaneebetweenlayerlandtheresistance plate 3, the beam of electrons passes almost directly from the layer to the plate with practically no. lateral spread and through that plate to the photo-emissive coating I, each elemental area of which is therebyactivated in succession to emit .electrons to the more positive electrode or anode 2.

As each elemental are. of the photo-emissive coating 4 is supplied with radiations corresponding -to the brightness of a corresponding'elemenelectron emission from this area of the coating is substantiallyproportional to the tone'value of the objectA over that area. This is due to A the fact that the high lateral resistance of the supporting plate t and ofthe photo-emissive coat-- ing 4 will prevent appreciable now ofement fromany area other than that directly'in front of the electron beam, which occupies a position corresponding to that of the moving beam of radiations B used to scan the photo-emissivelayer 1. In order to insure that the electron emission from the coating l shall be proportional to the intensity of: the-radiations renected from the elemental areas of the object or held of view, the intensity of the moving beam of` radiations B should be such that the electronic emission from the photo-emissive layer l, as the spot moves over it. is alwaysgreater than the-emission from any of the corresponding areas ofthe photoemissive coating I due to the radiations renected from the object.

It will thus be-seen that the moving beam of radiations B operates to control lthe liow of electrons within the scanning device in such manproportional to the tone values of corresponding elemental areas-of the object or field of view, are suppiiedto the external circuit, and hence an imags'current is caused to now through resistance l.

After being raised to the desired power level tal area oi the object or ileld of view O. the.

be used. as, for example, thatI disclosed4 in U. 8. Patent 1.999.316. issued to n. er.' stoner on april The television transmitting and receiving system shown in Fig. 4 differs from that described above mainly in regard to the means provided at the transmitter for producing the moving beam of radiations for scanning the photo-emissive layerlof-thescanningmemberMandalsoas to the iwpe of image producer used at the receiving station. It is lo be understood that the transmitterofli'ig.1 maybeusedwiththere- `ceiver o! Fig. 4 or the transmitter of Fig. 4 uses in a system which used a receiver similarto that described in connection with Fig. l.

The transmitter T' of the system shown in ll'lg. 4 comprbes a cathode ray tube Il of any well-known design which is substituted for the l elementsi'othe right-ofsectionlineMinFig. l.

Title cathode beam of the tube Il is deected in two directions at right anglesto each other. and at such relative speeds in the two directions that` its fluorescent screen 8| is activated to produce a moving spot of light which preferably moves across thescreen from onesidetotheotherin successive parallel lines and to completely traversethe screenfromtoptobottominatimeinterval within the period of persistence oi vision.

Dedection of the beam in one direction is ei'.

fectedbymeansoftheileldbetweenapairof denecting plates P produced by current having' a saw-tooth wave form supplied to the plates, the production of this current being controlled by oscillations of line scanning frequency supplied by the oscillator 32 to the deflection-current proner that successive current impulses. respectively by the ,device Ail, which may comprise a multi-je ducing device Il. Detection in the other' direction is eii'ected by supplying a second pair oi' denecting platesPi with a second current of sawtooth wave form supplied by the deileetion-current producing device I4, controlled by oscillations of image cycle frequency produced by a sub-harmonic generator 8l supplied with oscillations from theoscillator'n. Any suitable apparatus, .such as thatl disclosed in U. S.v Patent v 1,813,954. January 11, 192'I,-to Knoop, may be used to produce the deiiectingcurrents. l

passasse' :radiationsfromcammini;apotczushtcn` the iinorescentscreen are directed bythe lens systemgenerallyretedbyelens Il through thetplateGandthemetalliclayerltoscansutheelementalV case the image current isdirectly transmitted,-

or a demodulator and ampliiier in case sioniseiiectedinaccordancewithlinecarrler or radio practice. It also includes a cathode ray discharge device I6 comprising a cathode I1 and an anode I8 for producing the cathode ray beam. two pairs of plates II and 4l for respectively eiiecting deiiectlon of the beam in two In other mrds, thetwo sets o! elements torrespectively controlling the deilection and intensity oithebeanteachofwhichtends tointerierewith the other and thereby cause loss of focus of the beam and distortion of theimage, are so posi- -tionedand electrically controlled as to materially,

i! not completely. avoid such interference. For a more complete disclosure oi the construction and method of operation of the image producing cathode ray discharge device JB, briefly described above. reference may be made to Patent 2,155,-

directions at right angles to each other. a uorescentscreen4l,andapairoicontrolgrids42 and 4I connected by an external circuit 44 to which the image -currents received over line L and amplied by the device 45 are supplied by means oi' a transformer 4B. Auxiliary apparatus 33, 34 and 35. similar to that described in connection with the transmitter T' are also used in the receiving system R'. Devices and 34 operate, as disclosed in the above-mentioned Knoop patent, to supply deiiecting current of saw-tooth wave form to respective pairs oi deilecting plates 39 and 40. The apparatus tor producing the deiie'cting current is controlled by current of line scanning frequency received from the oscillator 82 at the transmitting station over a transmission line L'. In this manner. the cathode ray beam of the receiver R' is deectedin synchronism and in phase with the deflection of the cathode ray beam oi' the cathode ray discharge device 30 used for producing the scanning spot of light at the transmitter T'.

The cathode ray discharge device 38 operates in the following manner:

The control electrodes or grids 42 and 43 are closely adjacent each other in a position between the anode 38 and the iiuorescentl screen 4l and preferably comprise segments of a sphere, the centers oi which are close to the centers of the deilecting iields produced by the plates 39A and 40. The grid 42 is maintained at substantially the same potential as the anode 3B and thesignal potentials are applied to the grid 43 which may be negatively polarized with respect to the grid ,42.

The two grids, thereiore.'serve to denne a very limited zone in which the signal potentials are eiiective for controlling the cathode ray beam, and this zone is substantially isolated irom the equipotential section established between the anode 38 and grid 42, within which zone deection oi the beam is effected. In this manner, deiiection of the beam is controlled by the iield established 'between the pairs of plates 39 and 40 without being influenced by thessignal potentials, and the signal potentials operate to control the velocity or number of electrons constituting the beam and hence the intensity of the excitation of the uorescent screen, which determines the quality'of the image produced, without causing the direction of travel of the electrons to be varied.

192; issued April 18, 1939, to J. B. Johnson.

In each of the systems described in connection withFig. 1 and Fig. 4, there is provided a scan- `ning tube S enclosing a screen adapted to be i1- luminated with radiations received from an object or ileld of view, an associated electrode, and a scanning member, combined with simple optical means :or rendering the scanning member operative to effect scanning of the screen, and thereby control the production of an image current vary# ing with the light-tone values o! the object or iield of view.

The radiations applied to the object and those constituting the scanning beam supplied by the source I3 or the cathode ray discharge device 39 may be wholly within, partly withinvand partly without, or wholly outside the visible portion of the spectrum. When invisible rays are used. suitable lenses and glass will be used so as to pass these radiations. It may also be advisable in such a situation to provide appropriate iilters to exclude all except the radiations of the desired wave-length range. The object may be a human being or beings, an out-of-doors study. a motion picture illmor any other suitable field of view.

Other changes may be made without departing from the spirit or principles of the invention as hereinbefore expressed, `the scope of which is deiined by the appended claims.

What is claimed is:

1. -In an electro-optical system, an evacuated l vessel enclosing a thin, high resistance conduct.

ing plate with a photoemissive surface coating.

means for 'projecting an image 'oi an object onl said coating to cause emission of electrons theresurface of said light transmitting element into said space it will pass to the geometrically opposite elemental area oi said iirst plate without much spread. and means for scanning said second photo-emissive surface with a spot of light.

2. The combination with means for producing a moving beam of light, oi a separate unitary structure comprising a gas-tight container having an element therein for receiving light from said traveling iight beam and thereby setting up a beam oi low-velocity electrons having a corresponding movement in a continuation of the path of the light beam, and a second element upon which said 'beam of lowfvelocity electrons impinges element by element made of poorly conf ducting material, said second element being generally parallel to said first-mentioned element and spaced therefrom by a distance which is small compared with any dimension of said iirstmentioned element in a plane parallel to its face.

3. An image current producer comprising an image iiluminatedscreen comprising a thin, high resistance conducting plate with a coating there-- on oiv photo-emissive material, meansior scanning said screen with a beam ot low-velocity electrons, and means for producing a moving beam 'oi high velocity electrons for controlling the scanning of said screen by said low-velocity eleca moving beam of light for activating successively the, elemental areas o! thephoto-emissive layer ot saidscanning member for controlling the iicw 4. An electro-optical transmitter comprising a screen including a thin, high resistance conduct- 111gy Plate with a coating thereon o! photo-emissive material and means adlacent said screen lor generating a beam oi low-velocity electrons for scanning it, means i'or activating said screen with radiationscontroiled by anobject or iield of view. and means ior producing a moving beam-of light to control the operation or said iirst mentioned 5. A scanning apparatus for electro-optical systemscomprising a thin. opaque. high resistance `conducting plate adaptedto transmit current a thin, opaque. high resistance conducting plate adapted to transmit current transversely there- `through with substantially no lateral spread, a

photo-emissive coating li'or said plate, a scanning control member comprising a transparent plate. a transparent metallic layer, and a layer of photoemisslve material, an anode, and circuit connections between said anode and said transparent metallic layer.

.'I. -A scanning apparatus for electro-optical systems comprising an evacuated container enclosing `a thin. opaque, high resistance conductingrplate adapted to transmit current transversely there.

through with substantially no lateral spread, a'- photo-emissive coating for said plate, a scanning control member comprising a transparent plate. a transparent metallic layer, and a layer oi' photoemiasive material. and an anode electrode, means ior projecting au image' of an object upon the photo-emissive coating oi said opaque plate, andv means' producing a moving beam oi light for activating successively the elemental areas oi the photo-emissive layer o! said scanning member for controlling the iiow of electrons tcsaidanode.

8. A scanning apparatus tor electro-optical systems comprising an evacuated container enclosing a compositescreen or target comprising a thin, opaque. high resistance conducting plate whose lateral conductivity is negligible with respect to its transverse conductivity, a photo-emisaive coating for said opaque plate, a scanning control member comprising a -transparent. supporting plate. a thin, transparent metalliclayer, and a' Athxesecondofsaidelementscompri'sing'athiri` Y ond element, a source of potential, a resistance;` means for connecting the metallic layer oi said ance,andmeansiorconnectingsai'iihigli layer ot photo-sensitive material. and an anode.

circuit connections betweenA said anode and said transparent metallic laver so that-the anode has a positive potential with respect in said-scanning member, means for imparting a potential intermediate that oiv said anode and said scanning member to said opaque plate, means for projecting l an image oi an object upon the photo-emissive coating oisaid opaqusplataandmeansproducing of a moving beam ot electrons to said anode. s

9. A scanning apparatus for electro-optical syt-k tems comprising a gas-tight container enclosing three elements substantially parallel to and spaced apart from each other, the iirst o! said elements comprising a light transmitting member, the second of said elements comprising a thin opaque v high resistance conducting plate adapted to transmit current-transversely therethrough with substantially no lateral spread and a photo-emissive coating vfor said plate. said coating being adjacent said iirst element, and the third element comprising a transparent supporting plate, a metaliic layer capable oi transmitting light. and a laver o! photo-emiaa'ive material, said layer o! photo-emissive material being adjacent said second element.

`l0. A scanning apparatus for electro-optical.

systems comprising a gas-tight container enclosing three elements substantially parallel to and spaced apart fromeach other, the first oi'said elements comprising a light transmitting mem-, bei', the second of said elements comprising a thin .opaque high resistance conducting plata adapted to transmit current transversely therethrough with substantially no lateral spread and aphoto-emissive coating lor said plate. said coating being adjacent said nrst element, the third element comprising a transparent support lng Plate, a metallic layer capable ot light. and a layer of photo-emissive material. said layer oi photo-emissive material'being adjacent saidsecondelement,asourceotpotential.are

'sistanca means for connecting the'metallic layero! said third element to the negative 901e oi said source oi potential. and means for connecting said 'first element through said resistance to' the positive pole oi.' said source oi' potential.

` 41l. A scanning apparatus for electro-optical systems comprising a gas-tight container enclosing three elements aubstantially'parallel to and spaced apartiromeachothenthenrstoisaidelementscompriaing a .light transmitting member.

elementthroughsaidresistance poleofsaid so'irce of potential. a

ance conducting plate of ,said 'second element negative poles.

' .12. A scanning apparatus for electro-optical resistance conducting plate adeptsd itransmit current therethrough with substantially no lateral spread and a photo-emissive coating for said plate, said coatn ing being adjacent said rst element, andthe third element comprising a `transparent supporting plate, a metallic layer capable of transmitting light, and a layer ciphoto-emissive material, said layer of photo-emissiv'e material being adjacent said second element, a source of potential, a resistance, means for connecting the metallic layer of said third element to the negative pole of said source of` potential, means for connecting the said iirst element through said resistance to said positive pole of said source of potential, a second resistance, means for connecting said high resist- -ance conducting plate of said second element through said second resistance to apoint on 'said source of potential intermediate said positive and negative poles, means for projecting an image of lau. object upon the photo-emissive coating of said second element through said first element, and

layer.

FRANK GRAY. 

